[lib] Shim encrypted multimedia

Summary:
Linear task: ENG-1335

Added a condition to shim messages with types encrypted_{photo,video} as UNSUPPORTED. Added a note how to unshim them when supported.

Modified robotext to display new media types correctly.

Depends on D7168

Test Plan: Modified keyserver's upload-fetchers.js to send media messages with type encrypted_photo. On native wiped state and ensured that robotext is displayed.

Reviewers: atul, marcin, ashoat, rohan

Reviewed By: ashoat, rohan

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7169